[BUG] Claude Agent OAuth sign-in fails in Xcode 26.3 for Enterprise SSO — browser completes but Xcode never receives callback

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Claude Agent OAuth sign-in in Xcode 26.3 fails for Enterprise SSO accounts. The OAuth flow completes successfully in the browser (shows "Build something great. You're all set up for Claude Code"), but the callback never reaches Xcode. The spinner continues indefinitely.
Related to #10237 (now closed), issue remains unresolved.
Environment:

macOS 26.2 (Tahoe), Apple Silicon M4
Xcode 26.3
Claude Enterprise plan with SSO

What works:

Claude Sonnet 4 chat in Xcode (same account) ✅
Claude Code CLI in Terminal (same credentials) ✅

What doesn't work:

Claude Agent OAuth — callback never reaches Xcode ❌
Default workspace API key — rejected ❌

What Should Happen?

After completing OAuth in the browser, Xcode should receive the callback and authenticate Claude Agent. The Agent should work the same way Claude Sonnet 4 chat already works in Xcode with the same Enterprise account.

Error Messages/Logs

No error message displayed — Xcode shows an infinite spinner. Browser shows successful authentication ("Build something great. You're all set up for Claude Code.")

Steps to Reproduce

• Open Xcode 26.3 → Settings → Intelligence → Anthropic
• Click "Get" for Claude Agent (if not already installed)
• Click sign in with Claude.ai account (Enterprise SSO)
• Browser opens, complete SSO authentication
• Browser shows "Build something great. You're all set up for Claude Code."
• Return to Xcode — spinner continues indefinitely, sign-in never completes

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.77 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_